MRI BIRDCAGE COIL WITH DISTRIBUTED EXCITATION

Abstract

A magnetic resonance radio frequency transmission device (140) for generating and applying a radio frequency excitation field B.sub.1 for the purpose of magnetic resonance examination comprises a birdcage coil (144) and a plurality of M radio frequency amplifier units for providing radio frequency power at a magnetic resonance frequency to the birdcage coil (144) via a plurality of M activation ports (158) selected out of the plurality of N activation ports (158). In an operational state of the birdcage coil (144) each radio frequency amplifier unit (142) is electrically connected and is arranged in close proximity to an activation port (158). Among the plurality of M radio frequency amplifier units (142), there is established a fixed relationship of adjustable phase angles (φ) of the magnetic resonance radio frequency power provided by the plurality of M radio frequency amplifier units (142); a method of generating and applying a radio frequency excitation field B for the purpose of magnetic resonance examination, using such magnetic resonance radio frequency transmission device (140); and a magnetic resonance imaging system (110) configured for acquiring magnetic resonance images of at least a portion of a subject of interest (120), comprising such magnetic resonance radio frequency transmission device (140).

Claims

1. A magnetic resonance radio frequency transmission device for generating and applying a radio frequency excitation field B.sub.1 of a magnetic resonance frequency to nuclei of or within a subject of interest for the purpose of magnetic resonance examination, the magnetic resonance radio frequency transmission device comprising: a birdcage coil including: a pair of conductive loop members spaced along a common longitudinal axis, a plurality of N conductive segments aligned parallel to an axial direction and electrically interconnecting the conductive loop members, and a plurality of N activation ports, each activation port being configured to receive radio frequency power and to transfer the received radio frequency power to the birdcage coil for generating a portion of the radio frequency excitation field B.sub.1, wherein the plurality of N activation ports is located in close proximity to the birdcage coil; wherein the birdcage coil is designed to be excitable at at least N/2 separate resonance frequencies, the magnetic resonance radio frequency transmission device further including: a plurality of M radio frequency amplifier units, each radio frequency amplifier unit being configured for receiving radio frequency power from a radio frequency source and for amplifying the received radio frequency power, for providing radio frequency power at a magnetic resonance frequency to the birdcage coil via a plurality of M activation ports selected out of the plurality of N activation ports, wherein M is less than or equal to N, and wherein each radio frequency amplifier unit of the plurality of M radio frequency amplifier units configured to provide an individually adjustable level of radio frequency power at the magnetic resonance frequency to one activation port of the plurality of M activation ports; wherein, in an operational state of the birdcage coil, each radio frequency amplifier unit of the plurality of M radio frequency amplifier units is electrically connected to an activation port of the plurality of N activation ports by a radio frequency transmission line and is arranged in close proximity to the activation port which it provides radio frequency power; and wherein, in the operational state of the birdcage coil, among the plurality of M radio frequency amplifier units there is established a fixed relationship of adjustable phase angles and adjustable magnitudes of the magnetic resonance radio frequency power provided by the plurality of M radio frequency amplifier units and the magnetic resonance radio frequency transmission device comprising another radio frequency transmission line having an effective electrical length corresponding to substantially one wavelength of the magnetic resonance frequency, wherein two ends of the radio frequency transmission line are electrically connected to form a loop, a plurality of N contact points, electrically contacting the radio frequency transmission line and being arranged along the radio frequency transmission line in a spaced manner, wherein at least two contact points of the plurality of N contact points are each configured for receiving radio frequency power from at least one radio frequency source; wherein the number of radio frequency amplifier units of the plurality of M radio frequency amplifier units equals the number of activation ports of the plurality of N activation ports, and wherein each contact point of the plurality of N contact points is electrically connected to a different radio frequency amplifier unit of the plurality of M=N radio frequency amplifier units.

2. The magnetic resonance radio frequency transmission device as claimed in claim 1, wherein N is an integer multiple of M.

3. The magnetic resonance radio frequency transmission device as claimed in claim 1, wherein the conductive segments of the plurality of N conductive segments aligned parallel to the axial direction are equidistantly arranged with respect to an azimuthal direction about the common longitudinal axis of the birdcage coil.

4. The magnetic resonance radio frequency transmission device as claimed in claim 1, further comprising a control unit that is configured to control the individually adjustable level of radio frequency power provided at the magnetic resonance frequency by the plurality of M radio frequency amplifier units, and/or to control the fixed relationship of adjustable phase angles of the magnetic resonance radio frequency power among the plurality of M radio frequency amplifier units.

5. The magnetic resonance radio frequency transmission device as claimed claim 4, wherein the control comprises a digital wave generator that is configured for digitally establishing the fixed relationship of adjustable phase angles among the plurality of M radio frequency amplifier units and for setting the individually adjustable levels of radio frequency power at the magnetic resonance frequency at the plurality of M radio frequency amplifier units.

6. The magnetic resonance radio frequency transmission device as claimed claim 5, wherein the digital wave generator is based on a field programmable gate array.

7. The magnetic resonance radio frequency transmission device as claimed in claim 1, wherein the fixed relationship of adjustable phase angles and the individually adjustable levels of radio frequency power at the magnetic resonance frequency of the plurality of M radio frequency amplifier units are set for operating the birdcage coil in the quadrature mode.

8. A magnetic resonance imaging system configured for acquiring magnetic resonance images of at least a portion of a subject of interest, comprising: an examination space provided to position at least the portion of the subject of interest within; a main magnet provided for generating a static magnetic field Bo at least in the examination space; a magnetic gradient coil system configured for generating gradient magnetic fields superimposed to the static magnetic field B.sub.0; at least one magnetic resonance radio frequency transmission device as claimed in claim 1; at least one radio frequency antenna device that is configured for receiving magnetic resonance signals from the nuclei of or within the portion of the subject of interest that have been excited by applying the radio frequency excitation field B.sub.1; a control unit configured for controlling functions of the magnetic resonance imaging system; and a signal processing unit configured for processing magnetic resonance signals to determine magnetic resonance images of at least the portion of the subject of interest from the received magnetic resonance signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0061] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.

[0062] In the drawings:

[0063] FIG. 1 shows a schematic illustration of a part of an embodiment of a magnetic resonance imaging system in accordance with the invention,

[0064] FIG. 2 schematically illustrates the birdcage coil of the magnetic resonance radio frequency transmission device of the magnetic resonance imaging system pursuant to FIG. 1,

[0065] FIG. 3 schematically illustrates the embodiment of a the magnetic resonance radio frequency transmission device in accordance of the magnetic resonance imaging system pursuant to FIG. 1, providing radio frequency power to the birdcage coil shown in FIG. 2,

[0066] FIG. 4 depicts an alternative embodiment of a magnetic resonance radio frequency transmission device in accordance with the invention,

[0067] FIG. 5 is a schematic illustration of a digital wave generator of the control unit of the magnetic resonance imaging system pursuant to FIG. 1,

[0068] FIG. 6 is a schematic illustration of another alternative embodiment of a magnetic resonance radio frequency transmission device in accordance with the invention, and

[0069] FIG. 7 schematically illustrates a typical conventional configuration of radio frequency amplifiers providing radio frequency power to a birdcage coil for quadrature mode excitation.

DETAILED DESCRIPTION OF EMBODIMENTS

[0070] In the following, several embodiments in accordance with the invention are disclosed. The individual embodiments are described with reference to a particular figure and are identified by a prefix number of the particular embodiment. Features whose function is the same or basically the same in all embodiments are identified by reference numbers made up of the prefix number of the embodiment to which it relates, followed by the number of the feature. If a feature of an embodiment is not described in the corresponding figure depiction, or a reference number mentioned in a figure depiction is not shown in the figure itself, the description of a preceding embodiment should be referred to.

[0071] FIG. 1 shows a schematic illustration of a part of an embodiment of a magnetic resonance imaging system 110 configured for acquiring magnetic resonance images of at least a portion of a subject of interest 120, usually a patient, in accordance with the invention. The magnetic resonance imaging system 110 comprises a scanner unit 112 having a main magnet 114. The main magnet 114 has a central bore that provides an examination space 116 around a center axis 118 for the subject of interest 120 to be positioned within, and is further provided for generating a static magnetic field B.sub.0 at least in the examination space 116. For clarity reasons, a customary table for supporting the subject of interest 120 has been omitted in FIG. 1. The static magnetic field B.sub.0 defines an axial direction of the examination space 116, aligned in parallel to the center axis 118. It is appreciated that the invention is also applicable to any other type of magnetic resonance imaging systems providing an examination region within a static magnetic field.

[0072] Further, the magnetic resonance imaging system 110 comprises a magnetic gradient coil system 122 configured for generating gradient magnetic fields superimposed to the static magnetic field B.sub.0. The magnetic gradient coil system 122 is concentrically arranged within the bore of the main magnet 114.

[0073] The magnetic resonance imaging system 110 comprises a control unit 126 configured to control functions of the magnetic resonance imaging system 110. The control unit 126 includes a human interface device 124 formed by a monitor unit having a touch-sensitive screen.

[0074] Furthermore, the magnetic resonance imaging system 110 includes a magnetic resonance radio frequency transmission device 140 for generating and applying a radio frequency excitation field B.sub.1 of a magnetic resonance frequency during radio frequency transmit time periods to nuclei of or within the subject of interest 120 for the purpose of magnetic resonance examination. The magnetic resonance radio frequency transmission device 140 comprises a birdcage coil 144 designed as a whole-body coil and a plurality of four radio frequency amplifier units 142.sub.1-142.sub.4 (FIG. 3). The birdcage coil 144 has a center axis 146 and, in the operational state, is arranged concentrically within the bore of the main magnet 114 such that the center axis 146 of the birdcage coil 144 and the center axis 118 of the scanner unit 112 coincide (FIG. 1).

[0075] Each radio frequency amplifier unit 142 of the plurality of radio frequency amplifier units 142.sub.1-142.sub.4 is configured for receiving radio frequency power from a radio frequency source, for amplifying the received radio frequency power, and for providing the amplified radio frequency power of the magnetic resonance frequency to the birdcage coil 144.

[0076] To this end, radio frequency power is fed, controlled by the control unit 126, from a radio frequency transmitter 136 as the radio frequency source to the plurality of radio frequency amplifier units 142.sub.1-142.sub.4 (shown in FIG. 1 exemplarily for radio frequency amplifier unit 142.sub.1).

[0077] As is well known in the art, a cylindrical, metallic radio frequency shield 132 is arranged concentrically between the magnetic gradient coil system 122 and the birdcage coil 144.

[0078] Moreover, the magnetic resonance imaging system 110 comprises a plurality of radio frequency antenna devices 134 provided for receiving magnetic resonance signals from the nuclei of or within the subject of interest 120 that have been excited by applying the radio frequency excitation field B.sub.1. The radio frequency antenna devices 134 are designed as an array of local coils that are intended to be positioned proximal to a region of the subject of interest 120 to be imaged. The local coils are configured for receiving magnetic resonance signals from the excited nuclei of or within the portion of the subject of interest 120 to be imaged during radio frequency receiving time periods which are distinct from the radio frequency transmit time periods.

[0079] Furthermore, the magnetic resonance imaging system 110 comprises a signal processing unit 130 configured for processing magnetic resonance signals to determine magnetic resonance images of at least the portion of the subject of interest 120 from the received magnetic resonance signals.

[0080] A schematic view of the birdcage coil 144 of the magnetic resonance radio frequency transmission device 140 is given in FIG. 2. The birdcage coil 144 includes a pair of identical conductive loop members 148.sub.1, 148.sub.2 spaced along a common longitudinal axis given by the center axis 146, and a plurality of N=16 conductive segments 150.sub.1-150.sub.16 designed as straight conductors (rungs) that are aligned parallel to an axial direction. The birdcage coil 144 is designed as a band-pass type having a plurality of 2N loop capacitors 152.sub.1-152.sub.16, 154.sub.1-154.sub.16 placed in the conductive loop members 148.sub.1, 148.sub.2, in this way electrically interconnecting portions of the conductive loop members 148.sub.1, 148.sub.2 that form the conductive loop, and one rung capacitor 156.sub.i, i=1-N, of a plurality of N rung capacitors 156.sub.1-156.sub.16 placed in each of the rungs, which electrically interconnect the portions of the conductive loop members 148.sub.1, 148.sub.2 between two adjacent loop capacitors 152.sub.i, 152.sub.i+1, 154.sub.i, 154,.sub.i+1. The conductive segments 150.sub.i, i=1-N, of the plurality of N conductive segments 150.sub.1-150.sub.16 are equidistantly arranged with respect to an azimuthal direction 162 about the center axis 146.

[0081] The birdcage coil 144 is designed to be excitable at N/2 separate resonance frequencies. Methods of selecting birdcage coil design parameters for accomplishing this are known in the art and therefore need not be discussed in detail herein. The two resonant modes of the birdcage coil 144 considered for excitation are the ones whose radio frequency current distribution in the rungs is proportional to sin θ (mode 1) or cos θ (mode 2), respectively, wherein θ denotes the azimuthal angle measured in the azimuthal direction 162 about the center axis 146 of the birdcage coil 144.

[0082] According to the invention, the birdcage coil 144 comprises a plurality of N=16 activation ports 158.sub.1-158.sub.16. Each activation port 158.sub.i, i=1-16, is configured to receive radio frequency power and to transfer the received radio frequency power to the birdcage coil 144 for generating a portion of the radio frequency excitation field B.sub.1. The activation ports 158.sub.1-158.sub.16 of the plurality of N activation ports 158.sub.1-158.sub.16 are located in close proximity to the birdcage coil 144, which is best illustrated in an alternative embodiment of the magnetic resonance radio frequency transmission device shown in FIG. 4.

[0083] The magnetic resonance radio frequency transmission device 140 includes a plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4 (FIG. 3), which is less than the number N=16, which in turn is an integer multiple of M. A maximum power of each of the radio frequency amplifier units 142.sub.1-142.sub.4 is rated to be 4.4 kW, so that a total radio frequency power of about 18 kW is available for operating the birdcage coil 144. FIG. 3 schematically illustrates a configuration of the plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4, providing radio frequency power to the birdcage coil 144 via a plurality of M=4 activation ports selected out of the plurality of N=16 activation ports 158.sub.1-158.sub.16. The selected activation ports are activation ports 158.sub.3, 158.sub.7, 158.sub.11, 158.sub.15.

[0084] Each radio frequency amplifier unit 142.sub.i, i=1-4, of the plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4 is configured to provide an individually adjustable level of radio frequency power at the magnetic resonance frequency to one activation port 158 of the plurality of selected M=4 activation ports 158.sub.3, 158.sub.7, 158.sub.11, 158.sub.15.

[0085] In an operational state of the birdcage coil 144, each radio frequency amplifier unit 142.sub.i, i=1-M, of the plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4 is electrically connected to one activation port 158 of the selected activation ports 158.sub.3, 158.sub.7, 158.sub.11, 158.sub.15 of the plurality of N=16 activation ports 158.sub.1-158.sub.16 by a radio frequency transmission line 160.sub.i, i=1-4, having a length of less than one tenth of the effective wavelength of the resonance frequency in the radio frequency transmission line 160, and is thus arranged in close proximity to the activation port 158 to which it provides radio frequency power. In this embodiment of the magnetic resonance system 110 having a field strength of the static magnetic field B.sub.0 of 3.0 T, which results in a magnetic resonance frequency of about 128 MHz for protons, and a velocity factor of the transmission line of 66%, the length of the transmission lines 160.sub.1-160.sub.4 is less than about 0.15 m.

[0086] In the operational state, the birdcage coil 144 is driven by the plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4, providing magnetic resonance radio frequency power to the activation ports 158.sub.3, 158.sub.7, 158.sub.11, 158.sub.15 having amplitudes and relative phase angles φ.sub.3, φ.sub.7, φ.sub.11, φ.sub.15 according to (as customary, ω denotes the circular frequency of the magnetic resonance frequency)

activation port 158.sub.3: −½.Math.a.sub.11.Math.cos(ωt)+½.Math.b.sub.11.Math.sin(ωt)

activation port 158.sub.7: ½.Math.a.sub.7.Math.cos(ωt)−½.Math.b.sub.7.Math.sin(ωt)

activation port 158.sub.11: ½.Math.a.sub.11.Math.cos(ωt)−½.Math.b.sub.11.Math.sin(ωt)

activation port 158.sub.15: −½.Math.a.sub.7.Math.cos(ωt)+½.Math.b.sub.7.Math.sin(ωt)  (2)

[0087] In this way, activation ports 158.sub.7, 158.sub.15 are provided with radio frequency power having a phase shift of 180°. The same holds for activation ports 158.sub.3, 158.sub.11.

[0088] The four amplitudes a.sub.7, a.sub.11, b.sub.7 and b.sub.11 are independent and can be selected in order to accomplish a desired radio frequency excitation field B.sub.1 by driving the birdcage coil 144 accordingly.

[0089] For instance, quadrature operation of the birdcage coil 144 can be accomplished by selecting amplitudes b.sub.7 and a.sub.11 as 0, and selecting amplitude a.sub.7 to be equal to amplitude b.sub.11. Pairs of activation ports 158.sub.3/158.sub.7 and 158.sub.11/158.sub.15, respectively, which are separated by an azimuthal angle Δθ of 90°, measured in the azimuthal direction 162 about the center axis 146 of the birdcage coil 144, are provided with radio frequency power having a relative difference of the phase angle of 90°.

[0090] As shown above, in the operational state of the birdcage coil 144, among the plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4 there is established a fixed relationship of adjustable phase angles φ.sub.i, i=1-4, of the magnetic resonance radio frequency power provided by the plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4.

[0091] For radio frequency shimming purposes, the four independent amplitudes a.sub.7, a.sub.11, b.sub.7 and b.sub.11 can be selected differently, for instance for compensating inhomogeneity of the radio frequency excitation field B.sub.1 caused by the subject of interest 120. The birdcage coil 144 is then operated as a dual radio frequency channel multi-transmission coil with M amplifiers.

[0092] The individually adjustable level of radio frequency power provided at the magnetic resonance frequency by the plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4 and the fixed relationship of adjustable phase angles φ.sub.i, i=1-4, of the magnetic resonance radio frequency power among the plurality of M=4 radio frequency amplifier units 142.sub.1-142.sub.4 is controlled by a control unit that is an integral part of the control unit 126 of the magnetic resonance imaging system 110.

[0093] An alternative embodiment of a magnetic resonance radio frequency transmission device 240 in accordance with the invention is schematically illustrated in FIG. 4. Only features differing from the embodiment pursuant to FIG. 3 will be described. For features of the second embodiment that are not described hereinafter, reference is made to the description of the first embodiment.

[0094] In comparison to the first embodiment (FIG. 3), the embodiment of a magnetic resonance radio frequency transmission device 240 pursuant to FIG. 4 comprises a plurality of M=N=16 radio frequency amplifier units 242.sub.1-242.sub.16, so that the number M of radio frequency amplifier units 242.sub.1-242.sub.16 is equal to the number N of rungs of the birdcage coil 244.

[0095] A maximum power of each radio frequency amplifier unit 242.sub.ii=1-16, of the plurality of the radio frequency amplifier units 242.sub.1-242.sub.16 is rated to be 1.1 kW only. Nonetheless, a total radio frequency power of about 18 kW is still available for operating the birdcage coil 244. FIG. 4 schematically illustrates a configuration of the plurality of M=16 radio frequency amplifier units 242.sub.1-242.sub.16 providing radio frequency power to the birdcage coil 244 via a plurality of M=N=16 activation ports 258.sub.1-258.sub.16.

[0096] The radio frequency amplifier units 242.sub.1-242.sub.16 are conveniently arranged on a flange 238 of the main magnet 214, in close proximity to the activation ports 258.sub.1-258.sub.16, and each radio frequency amplifier unit 242.sub.i, i=1-16, of the plurality of the radio frequency amplifier units 242.sub.1-242.sub.16 is electrically connected to one activation port 258.sub.i, i=1-16, of the plurality of activation ports 258.sub.1-258.sub.16 by a frequency transmission line 260.sub.i, i=1-16, designed as a coaxial cable having a length of less than 0.15 m.

[0097] In the operational state, the birdcage coil 244 is driven by the plurality of M=N=16 radio frequency amplifier units 242.sub.1-242.sub.16 providing magnetic resonance radio frequency power to the plurality of N=16 activation ports 258.sub.1-258.sub.16 having amplitudes and relative phase angles φ.sub.i, i=1-16, according to

[00002]$\begin{array}{cc}\cos \left(\omega .Math..Math.t\right).Math.\left(a_7.Math.\cos \left[\frac{2.Math.\pi }{N}.Math.\left(i-f\right)\right]+a_{11}.Math.\sin \left[\frac{2.Math.\pi }{N}.Math.\left(i-f\right)\right]\right)-\sin \left(\omega .Math..Math.t\right).Math.\left(b_7.Math.\cos \left[\frac{2.Math.\pi }{N}.Math.\left(i-f\right)\right]+b_{11}.Math.\sin \left[\frac{2.Math.\pi }{N}.Math.\left(i-f\right)\right]\right)& \left(3\right)\end{array}$

[0098] wherein index number i denotes a conductive segment 250.sub.i of the plurality of conductive segments 250.sub.1-250.sub.16 of the birdcage coil 244, running from 1 to N in the azimuthal direction 262 about the center axis 246, and index f denotes an activation port 258.sub.f at which an absolute phase angle is arbitrarily set to 0. In the above case, this is the case for activation port 258.sub.7.

[0099] Again, the four amplitudes a.sub.7, a.sub.11, b.sub.7 and b.sub.11 are independent and can be selected in order to accomplish a desired radio frequency excitation field B.sub.1 by driving the birdcage coil 244 accordingly.

[0100] For instance, quadrature operation of the birdcage coil 244 can be accomplished by selecting amplitudes b.sub.7 and a.sub.11 as 0, and selecting amplitude a.sub.7 to be equal to amplitude b.sub.11. The individually adjustable level of radio frequency power provided at the magnetic resonance frequency by the plurality of M=N radio frequency amplifier units 242.sub.1-242.sub.16 is then, according to (3), set to a.sub.7=b.sub.11, i.e. to a radio frequency power level that is substantially identical among the M=N radio frequency amplifier units 242.sub.1-242.sub.16. The fixed relationship of adjustable phase angles φ.sub.i, i=1-16, of the magnetic resonance radio frequency power among the plurality of M=N radio frequency amplifier units 242.sub.1-242.sub.16 is established according to

[00003]${\phi }_i=\frac{2.Math.\pi }{N}.Math.\left(i-f\right)$

[0101] From i=1 to N, phase angles φ.sub.i are equal to −135°, −112.5°, −90°, −67.5°, −45°, −22.5°, 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, 157.5°, 180° and −157.5°, respectively.

[0102] For radio frequency shimming purposes, the four independent amplitudes a.sub.7, b.sub.7 and b.sub.11 and the phase angles φ.sub.i, i=1-16, can be selected differently, for instance for compensating inhomogeneity of the radio frequency excitation field B.sub.1 caused by the subject of interest 220. The birdcage coil 244 is then operated as a 16-radio frequency channel multi-transmission coil.

[0103] FIG. 5 shows a schematic illustration of a module 66 of a digital wave generator 64 which is an integral part of the control unit 226 of the magnetic resonance imaging system 210. The digital wave generator 64 comprises a plurality of modules 66.sub.i, i=1-16, each of which is assignable to one of the radio frequency amplifier units 242, of the plurality of M=N radio frequency amplifier units 242.sub.1-242.sub.16. The control unit 226 controls the digital wave generator 64 that is based on a field programmable gate array, and that is configured for digitally establishing the fixed relationship of adjustable phase angles φ.sub.i, i=1-16, among the plurality of M=N radio frequency amplifier units 242.sub.1-242.sub.16 according to formula (3), and for setting the individually adjustable levels of radio frequency power at the magnetic resonance frequency at the plurality of M=N radio frequency amplifier units 242.sub.1-242.sub.16 according to formula (3).

[0104] Another alternative embodiment of a magnetic resonance radio frequency transmission device 340 in accordance with the invention is schematically illustrated in FIG. 6.

[0105] In comparison to the embodiment pursuant to FIG. 4, the magnetic resonance radio frequency transmission device 340 shown in FIG. 6 further includes another radio frequency transmission line 72 having an effective electrical length corresponding to substantially one wavelength of the magnetic resonance frequency. The two ends of the radio frequency transmission line 72 are electrically connected to form a loop.

[0106] The magnetic resonance radio frequency transmission device 340 further includes a plurality of N=16 contact points 74.sub.i, i=1-N, electrically contacting the radio frequency transmission line 72 and being arranged along the radio frequency transmission line 72 in a spaced manner, wherein contact points 74, of the plurality of N contact points 74 i=1-N, are each configured for receiving radio frequency power from two independent radio frequency sources 68, 70.

[0107] In the embodiment shown in FIG. 6, contact points 74.sub.7 and 74.sub.11 receive radio frequency power from the two independent radio frequency sources 68, 70.

[0108] The number M of radio frequency amplifier units 342.sub.i, i=1-M, of the plurality of M radio frequency amplifier units 342.sub.i, i=1-M, equals the number N of activation ports 358.sub.i, i=1-N, of the plurality of N activation ports 358.sub.i, i=1-N. Each contact point 74, of the plurality of N contact points 74.sub.i, i=1-N, is electrically connected to an input port of a different radio frequency amplifier unit 342, of the plurality of M=N radio frequency amplifier units 342.sub.i, i=1-16. The radio frequency path from contact point 74.sub.15 to radio frequency amplifier unit 342.sub.15 and further to activation port 358.sub.15 of birdcage coil 344 via transmission line 360.sub.15 is exemplarily and schematically indicated in FIG. 6.

[0109] In this way, the radio frequency power that is available at one of the N contact points 74.sub.i, i=1-N, is used as an input radio frequency power for one of the plurality of M=N radio frequency amplifier units 342.sub.i, i=1-16. A fixed relationship of adjustable phase angles φ.sub.i, i=1-16, and magnitudes of the magnetic resonance radio frequency power provided by the plurality of M=N radio frequency amplifier units 342.sub.i, i=1-16, is established by the radio frequency transmission line 72 in an analog way by the effective electrical length of portions of the radio frequency transmission line 72 between any two contact points 74.sub.m, 74.sub.n, m, n=1-16, wherein the effective electrical length between two adjacent contact points 74, 74.sub.n+1 for any n is equal to 360°/N.

[0110] Amplitudes and relative phase angles φ.sub.k, k=1, 2, of the two independent radio frequency sources 68, 70 are given by (cf. formula (2))

activation port 358.sub.7: ½.Math.a.sub.7.Math.cos(ωt)−½.Math.b.sub.7.Math.sin(ωt)

activation port 358.sub.11: ½.Math.a.sub.11.Math.cos(ωt)−½.Math.b.sub.11.Math.sin(ωt)

[0111] Quadrature operation of the birdcage coil 344 can be accomplished by selecting amplitudes b.sub.7 and a.sub.11 as 0 (which means a phase shift between the two independent radio frequency sources of (90°), selecting amplitude a.sub.7 to be equal to amplitude b.sub.11 and operating each radio frequency amplifier unit 342, of the plurality of M=N radio frequency amplifier units 342.sub.i, i=1-16, at the same gain, i.e. at the same individually adjustable level of radio frequency power.

[0112] For operating the birdcage coil 344 in a 2-radio frequency channel multi-transmission mode for the purpose of radio frequency shimming, the independent amplitudes a.sub.7, a.sub.11, b.sub.7 and b.sub.11 can be chosen as desired.

[0113] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

TABLE-US-00001 REFERENCE SYMBOL LIST: 10 magnetic resonance imaging system 12 scanner unit 14 main magnet 16 examination space 18 center axis 20 subject of interest 22 magnetic gradient coil system 24 human interface device 26 control unit 28 digital memory unit 30 signal processing unit 32 radio frequency shield 34 plurality of receiving radio frequency antenna devices 36 radio frequency transmitter 38 flange 40 magnetic resonance radio frequency transmission device 42 radio frequency amplifier unit 44 birdcage coil 46 center axis 48 conductive loop member 50 conductive segment 52 loop capacitor 54 loop capacitor 56 rung capacitor 58 activation port 60 radio frequency transmission line 62 azimuthal direction 64 digital wave generator 66 module 68 radio frequency source 70 radio frequency source 72 radio frequency transmission line 74 contact point φ phase angle θ azimuthal angle